The present invention relates to an optical pickup device that records data on and/or reproduces data from a plurality of types of optical discs of different recording densities, and to an objective lens for use with the optical pickup device.
With respect to the optical disc, a plurality of standards have been specified based on different recording densities. For example, optical discs according to new standards such as BD (Blu-ray Disc) and HD-DVD, recently put to practical use with a further increased storage capacity of information, are granted with an even higher recording density than the DVD (Digital Versatile Disc), which is one of the optical discs having a highest recording density among those commercially available. Accordingly, in order to secure the compatibility of an optical information recording and reproducing apparatus with the optical discs of different standards, the apparatus has to be able to change the numerical aperture (NA) of the light used for recording or reproducing the information, thus to obtain beam spots corresponding to different recording densities.
More specifically, for recording or reproducing information on or from the optical disc of the new standard, an optical system having a higher NA than a DVD-exclusive optical system has to be employed, so as to make the beam spot finer. Since the spot diameter can be diminished as the wavelength becomes shorter, the optical system that handles the optical disc according to the new standard has to be equipped with a laser source capable of generating an oscillation wavelength shorter than approx. 660 nm employed by the DVD-exclusive optical system, for example approx. 405 nm. Recently, therefore, optical information recording and reproducing apparatuses compatible with the optical discs of different standards are provided with an optical pick-up device including a light source unit that can output a laser beam in different oscillating wavelengths.
Also, it is known that an optical disc with a higher recording density generally has a more rigorous tolerance against aberration. Accordingly, minimizing spherical aberration is also a critical issue with optical information recording and reproducing apparatus compatible with the DVD and the optical disc according to the new standard.
It is to be noted that the term “optical information recording and reproducing apparatus” herein refers to all apparatuses including an apparatus exclusively for recording information, an apparatus exclusively for reproducing information, and an apparatus that can both record and reproduce information. Also, “compatibility” means that the information recording or reproducing function of the apparatus is maintained unchanged without replacing any part thereof, despite utilizing the optical discs of different standards.
For converging the laser beam on the recording surface of the optical discs of different standards without degrading the quality of information in all cases, it has been proposed to provide a diffracting structure in the optical path of the laser beam. Such proposal can be found, for example, in Japanese Patent Provisional Publications Nos. 2001-93179 (hereafter, referred to as JP 2001-93179A) and HEI 09-54973 (hereafter, referred to as JP H09-54973A).
The publication JP 2001-93179A discloses an optical pickup including a diffracting optical element that generates a laser beam in the level of 400 nm for recording or reproducing information on the optical disc of the new standard, and a laser beam in the level of 650 nm for recording or reproducing information on the DVD. The diffracting optical element in the optical pickup according to JP 2001-93179A utilizes second order diffracting light for recording or reproducing information on the optical disc of the new standard, and first order diffracting light for recording or reproducing information on the DVD. In another occasion, the diffracting optical element utilizes third order diffracting light for recording or reproducing information on the optical disc of the new standard, and the second order secondary diffracting light for recording or reproducing information on the DVD. Such arrangement allows correcting spherical aberration originating from the difference in wavelength of the laser beams, while restraining degradation in diffraction efficiency of the laser beams, thus to form optimal beam spots for recording or reproducing information on the recording surface of the respective optical discs.
The optical pickup according to JP 2001-93179A, however, does not refer to fluctuation of the spherical aberration arising from a minute shift in wavelength of the laser beam employed for the respective optical discs, from the wavelength specified as appropriate (hereinafter, “design wavelength”). When employing the diffracting optical element for attaining compatibility with the optical discs with a cover layer different in thickness, the fluctuation of the spherical aberration resultant from the minute shift in wavelength of the actually employed laser beam from the design wavelength turns out to be particularly prominent. The fluctuation of the spherical aberration may disturb the information recording or reproducing performance, particularly when recording or reproducing the information on the optical disc of the new standard, which has a higher recording density.
The publication JP H09-54973A discloses an optical head unit compatible with the two types of optical disc based on, as the JP 2001-93179A, the diffraction effect of the diffracting optical element. In the optical head unit, the diffracting optical element utilizes first order diffracting light for recording or reproducing information on the optical disc with a higher recording density, and a transmitted light recording or reproducing the information on the optical disc with a lower recording density.
Here, the optical head unit according to JP H09-54973A is only compatible with the DVD and CD (Compact Disc), which has a relatively lower recording density than the DVD. In other words, recording or reproducing the information on an optical disc of the new standard is out of the scope of the optical head unit according to JP H09-54973A. Accordingly, employing the laser beam of the wavelength level of 400 nm in the optical head unit according to JP H09-54973A, results in emergence of various aberrations on the recording surface of the optical disc of the new standard, including the spherical aberration, thus failing in forming a beam spot appropriate for recording or reproducing the information on the optical disc of the new standard. Therefore, the optical head unit according to JP H09-54973A is unable to record and reproduce the information on the optical disc of the new standards. In addition, the optical head unit according to JP H09-54973A does not provide any solution either, for controlling the spherical aberration originating from the minute shift of the laser beam utilized for the optical discs from the design wavelength. Consequently, the technique disclosed in P H09-54973A has the similar drawback to that of JP 2001-93179A.